The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a camera to be built in a portable device like a cellular phone and a portable information terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones”, i.e., multifunctional cellular phones which can run various application software as well as a voice call function, have been more widely used. When application software is run on smartphones, it is possible to achieve functions such as those of digital still cameras and car navigation systems on the smartphones. In order to achieve those various functions, most models of smartphones include cameras similarly to cellular phones.
Generally speaking, product groups of such smartphones are often composed according to specifications for beginners to advanced users. Among them, an imaging lens to be mounted in a product designed for the advanced users is required to have a high-resolution lens configuration so as to be also applicable to a high pixel count imaging element of these years, as well as a small size.
As a method of attaining the high-resolution imaging lens, there has been a method of increasing the number of lenses that compose the imaging lens. However, the increase of the number of lenses easily causes an increase in the size of the imaging lens, and is a disadvantageous for mounting in a small-sized camera such as the above-described smartphones. For this reason, for developing an imaging lens, it has been conventionally typical to focus on shortening a total track length, while attaining higher resolution of the imaging lens.
With rapid advancement in achieving the higher pixel count of an imaging element and image processing technology in these days, an imaging lens has been developed so as to attain higher resolution rather than a shorter total track length of the imaging lens. For example, conventionally, it has been typical to mount a camera unit, which includes an imaging lens and an imaging element, in the smartphone. There has also been an attempt to attach a separate camera unit onto a smartphone, whereby it is possible to obtain images equivalent to those taken by digital still cameras. In other words, space restriction upon mounting a camera in a smartphone is solved by configuring a camera separately from the smartphone.
In case of a lens configuration composed of six lenses, due to the large number of lenses of the imaging lens, it is somewhat difficult to reduce the size of the imaging lens. However, because of high flexibility in design, it has potential to attain satisfactory correction of aberrations and downsizing, which are necessary for high-resolution imaging lenses, in a balanced manner. For example, as the imaging lens having the six-lens configuration as described above, an imaging lens described in Patent Reference has been known.
Patent Reference: Japanese Patent Application Publication No. 2013-195587
The imaging lens described in Patent Reference includes a first lens that is positive and directs a convex surface thereof to an object side, a second lens that is negative and directs a concave surface thereof to an image plane side, a third lens that is negative and directs a concave surface thereof to the object side, a fourth and fifth lenses that are positive and direct convex surfaces thereof to the image plane side, and a sixth lens that is negative and directs a concave surface thereof to the object side. According to the conventional imaging lens of Patent Reference, by satisfying conditional expressions of a ratio between a focal length of the first lens and a focal length of the third lens and a ratio between a focal length of the second lens and a focal length of the whole lens system, it is achievable to satisfactorily correct a distortion and a chromatic aberration.
Each year, functions and sizes of cellular phones and smartphones are getting higher and smaller, and the level of a small size required for an imaging lens is even higher than before. In case of the imaging lens of Patent Reference, since a distance from an object-side surface of a first lens to an image plane of an imaging element is long, there is a limit by itself to achieve satisfactory correction of aberrations while downsizing the imaging lens to satisfy the above-described demands. It is achievable to reduce the level of downsizing required for an imaging lens by providing a camera as a separate unit from cellular phones or smartphones. However, in terms of convenience or portability, cellular phones or smartphones with built-in cameras are still dominantly preferred. Therefore, there remains such a strong demand for small imaging lenses with high resolution.
Here, such a problem is not specific to the imaging lens to be mounted in cellular phones and smartphones. Rather, it is a common problem even for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, vehicle onboard cameras, and network cameras.
In view of the above-described problems in the conventional techniques, an object of the present invention is to provide an imaging lens that can attain both downsizing thereof and satisfactory aberration correction.
Further objects and advantages of the present invention will be apparent from the following description of the invention.